1. Field of the Invention
The present invention relates to current sensing, an operation for detecting a current flowing through the bit line, and in particular to a current detection type sense circuit of a semiconductor.
2. Description of the Related Art
FIG. 1 is a diagram showing an example of a bit line selection circuit and a page buffer/sense circuit of a conventional flash memory. Here, a pair of bit lines are illustrated by an even-number bit line GBL_e and an odd-number bit line GBL_o. A bit line selection circuit 10 comprises an even-number selection transistor SEL_e connected to the even-number bit line GBL_e, an odd-number selection transistor SEL_o connected to the odd-number bit line GBL_o, an even-number bias selection transistor YSEL_e connected between the even-number bit line GBL_e and a virtual potential VIR, an odd-number bias selection transistor YSEL_o connected between the odd-number bit line GBL_o and the virtual potential VIR, and a bit line selection transistor BLS connected to a common node N1 of the even-number selection transistor SEL_e and the odd-number selection transistor SEL_o. An NAND string NU is connected between the even-number bit line GBL_e and a common source line SL, and an NAND string NU is connected between the odd-number bit line GBL_o and the common source line SL.
A sense circuit 20 comprises a pre-charge transistor BLPRE for supplying pre-charge potential to the bit lines, a capacitor C connected to a sense node SN that is located between the pre-charge transistor BLPRE and the bit line selection transistor BLS, and a pass transistor BLCD which transfers the potential of the sense node SN to a latch circuit 22.
When the even-number bit line GBL_e is selected and the odd-number bit line GBL_o is unselected, the even-number selection transistor SEL_e and the bit line selection transistor BLS are turned on and the odd-number selection transistor SEL_o is turned off. When the odd-number bit line GBL_o is selected and the even-number bit line GBL_e is unselected, the odd-number selection transistor SEL_o and the bit line selection transistor BLS are turned on and the even-number selection transistor SEL_e is turned off. In this way, one sense circuit 20 is shared by two bit lines GBL_e and GBL_o.
Patent Document 1 discloses the following techniques. Under a readout operation, when the even-number bit line GBL_e is selected, the even-number bias selection transistor YSEL_e is turned off, the odd-number bias selection transistor YSEL_o is turned on, and the odd-number bit line GBL_o is supplied with GND potential by the virtual potential VIR. Conversely, when odd-number bit line GBL_o is selected, the even-number bias selection transistor YSEL_e is turned on, the odd-number bias selection transistor YSEL_o is turned off, and the even-number bit line GBL_e is supplied with GND potential by the virtual potential VIR. In this way, when the even-number bit line is read out, the odd-number bit line will be supplied with GND potential, and when the odd-number bit line is read out, the even-number bit line will be supplied with GND potential. Thereby, bit line shielding is performed to reduce noise due to capacitive coupling of adjacent bit lines.
Patent Document 1: Patent publication no. JP11-176177
The sense circuit 20 shown in FIG. 1 is a so-called voltage detection type sense circuit, which supplies the pre-charge potential to the even-number bit line GBL_e or the odd number bit line GBL_o via the pre-charge transistor BLPRE and so on, then makes the bit line be discharged in response to the memory state of the selected memory cell, and detects the discharge state at the sense node SN.
However, such voltage detection type sense circuits make it difficult to reduce sensing time when the bit lines become a tiny structure. Namely, as the width or pitch of bit lines gets smaller, the resistance of the bit lines increases and the capacitive coupling between the bit lines also increases, resulting in it taking a long time to pre-charge a constant voltage to the bit lines. In particular, when a shielding readout operation is performed on a bit line, the adjacent bit line is shielded to be at GND potential. Thus, the pre-charge time for a selected bit line becomes longer.
On the other hand, the threshold values of erased memory cells have non-uniformity. Under a readout operation, current flowing through a bit line is not always constant. Therefore, even if a current detection type sense circuit were to be utilized to replace the voltage detection type sense circuit, the ability to determine little current flowing through the bit line would be required.
To solve the above problems, the purpose of the invention is to provide a current detection circuit which can rapidly detect current flowing through a tiny bit line structure. The purpose of the invention is also to provide a semiconductor apparatus having the current detection circuit.